The present invention relates to an organic light-emitting device.
Organic light-emitting devices generally act as a light source that perfectly diffuses light, thereby suffering remarkable loss in light quantity when they are used for liquid crystal projectors, optical communication devices, light-exposing devices of printers, etc. It is thus desired to develop an organic light-emitting device excellent in light emission directivity, and some proposals have been made therefor. For example, Japanese Patent Laid-Open No. 9-180883 disclosed a micro-optical resonator type organic light-emitting device, which controls emitting quantity of light other than those in directions perpendicular to a substrate to a low level, thereby exhibiting excellent light emission directivity. This micro-optical resonator type organic light-emitting device has a micro-optical resonator provided by disposing an organic layer between a conductive multi-layered mirror and a metal mirror, and has a resonance wavelength shorter than a peak wavelength in its emission spectrum.
In the above-mentioned micro-optical resonator type organic light-emitting device, it is desired that optical length thereof is as short as possible to obtain single mode resonance. Though the optical length of the device may be controlled by changing the thickness of the organic layer or the electrode of ITO (Indium Tin Oxide), etc., the organic layer cannot be changed so much in its thickness as to control the optical length to desirable level practically, so that the optical length is generally shortened by reducing the thickness of the electrode. However, when the thickness of the electrode is reduced, surface resistance of the electrode is unavoidably increased, whereby the organic light-emitting device comprising the electrode is poor in light-emitting characteristics, durability, etc.
Further, though Japanese Patent Laid-Open No. 11-288786 disclosed a micro-optical resonator type organic light-emitting device having optical length that is increased to lower surface resistance of ITO electrode, this device is disadvantageous in that multi mode resonance is often obtained.
An object of the present invention is to provide a micro-optical resonator type organic light-emitting device that scarcely causes multi mode resonance even if optical length thereof is increased by thickening a transparent electrode.
As a result of intense research in view of the above object, the inventor has found that a micro-optical resonator type organic light-emitting device comprising a multi-layered thin film having particular light reflectivity scarcely causes multi mode resonance. The present invention has been accomplished by the finding.
Thus, a micro-optical resonator type organic light-emitting device of the present invention comprises a substrate, and a multi-layered thin film, a transparent electrode, an organic light-emitting layer and a back electrode disposed on the substrate, and if 0-mode resonance wavelength of the device is expressed by xcex0, n-mode resonance wavelength of the device is expressed by xcexn (in which n is a positive integer) and reflectivity of the multi-layered thin film against light having a wavelength xcex is expressed by R(xcex), R(xcex0) is 40% or more and R(xcex1) or R(xcex2) is 30% or less.
In the micro-optical resonator type organic light-emitting device of the present invention, R(xcex0) is preferably 60% or more, more preferably 80% or more. R(xcex1) and R(xcex2) is preferably 30% or less. Further, it is more preferable that R(xcex1) and/or R(xcex2) is 20% or less, and it is particularly preferable that R(xcex1) and/or R(xcex2) is 10% or less.
The multi-layered thin film is preferably made of one or more material selected from the group consisting of SiO2, TiO2, Al2O3, MgF2 and ZrO2. The multi-layered thin film is preferably composed of eight or more layers. Optical length of the organic light-emitting device is particularly preferably four times or more as long as half of the wavelength xcex0. Further, the transparent electrode preferably has surface resistance of 50 xcexa9/square or less. Thickness of the transparent electrode is preferably 40 nm or more, more preferably 80 nm or more.